Amphiboles were synthesized at 750 degrees C, 1 kbar (H 2 O) for compositions at 20% intervals along the join potassium-richterite-tremolite. Structural variations, site occupancies, and modal analyses of the experimental products (amphibole+minor diopside, quartz, and enstatite) were characterized by Rietveld structure refinement, with final R Bragg indices in the range 4-6%, and by infrared spectroscopy in the principal OH-stretching region. Amphibole compositions were determined by (1) site-scattering refinement for the A and M4 sites that are occupied by (K,[]) ([] = vacancy) and (Na,Ca), respectively; and (2) mass-balance calculations involving the modal analysis and the nominal experimental product composition. These measurements agree within 1% absolute and show close agreement with electron-microprobe compositions for the two samples that we could analyze. Deviations from nominal amphibole composition are up to 19% absolute. The resulting relations between cell dimension and composition are linear. The major change in cell dimensions is a decrease of 0.25 Aa in a with increasing tremolite component. The infrared spectra show two principal peaks at 3735 and 3675 cm (super -1) , corresponding to the local arrangements MgMgMg-OH- A K (the Kr band) and MgMgMg-OH- A [] (the Tr band), respectively. The relative variation in peak intensity as a function of amphibole composition shows that the molar absorptivities of the two bands are significantly different. The ratio of the molar absorptivities for the two bands is 2.2.

You do not currently have access to this article.